This invention relates generally to support arms for surgical theater systems and more particularly to support arms having a counterbalance arm to balance the weight of the supported object so that it can be suspended at a selected height without locking the arm.
Surgical theater systems are floor, wall, or ceiling mounted systems including devices which can be positioned in desired locations within a surgical suite to maximize there accessibility, utility, and/or viewability. Examples of surgical light systems can be found in U.S. Pat. Nos. 6,012,821 and 6,132,062 and U.S. applications Ser. Nos. 09/050,265; 09/050,529; 09/050,534 and 09/747,605 which are expressly incorporated herein by reference. Such surgical theater systems support devices such as surgical lights, monitors, cameras, and electronic instruments utilized by surgeons and operating room personnel during the performance of surgical procedures. The above identified patents demonstrate that it is well known to provide surgical theater systems which facilitate the positioning, both horizontally and vertically, of the devices attached thereto to maximize their utility. In order to maintain the proper orientation in space of the device mounted to the counterbalance arm, the counterbalance arm includes a parallelogram linkage.
U.S. application Ser. No. 09/747,605 shows a surgical theater system with a counterbalance arm which seeks to minimize vertical movement of a device after it has been positioned at a desired height by providing a counterbalance arm having frictional brakes engaging pivot pins about which the arm rotates. Counterbalance arms are well known devices incorporating counterweights or spring mechanisms which provide a vertical upward force to the distal end of a cantilevered arm to counteract the downward force exerted on the distal end of the cantilevered arm by the object supported. The forces create torques about the lower pivot axis of the counterbalance arm. Ideally, the torque exerted on the counterbalance arm will be equal in magnitude to, but Opposite in direction to, the torque exerted on the counterbalance arm by the weight. Recognizing that the ideal situation cannot always be achieved, U.S. application Ser. No. 09/747,605 compensates for the inequalities by applying a frictional force to the pivot axis which permits the torques of the weight and the spring members to be slightly unbalanced.
The torque exerted by a weight hanging freely suspended from a fixed point on an arm is proportional to the sine of the angle formed between the vertical plane and the longitudinal axis of the arm. Thus the greatest torque is exerted by an object freely hanging from a fixed point on a beam when the longitudinal axis of the beam is horizontal, sin 90 deg=1.
Springs and gas springs typically obey Hooke's law, at least while in the region of elasticity, so that the force exerted by the spring is proportional to its displacement from equilibrium. In counterbalance arms, the spring is typically not mounted perpendicular to the longitudinal axis of the arm but rather couples to the distal end of the arm and to an end of a spring link. The spring link is typically pivotally mounted at a first end to a fixed location relative to the pivot axis of the proximal end of the counterbalance arm and mounted at the other end to slide within a slot formed in the counterbalance arm. The spring is typically pivotally mounted to the sliding end of the spring link and pivotally mounted to the second end of the counterbalance arm. Thus the spring exerts a force directed substantially along the longitudinal axis which is converted by the spring link into components of force parallel to and perpendicular to the longitudinal axis of the spring link. The component parallel to the longitudinal axis creates strain along the link which is assumed to be incompressible. The component perpendicular to the longitudinal axis of the spring link induces the pivot pin sliding in the slot into engagement with the upper edge of the slot so that the perpendicular component reduces to a component perpendicular to the longitudinal axis of the counterbalance arm and a component parallel to the longitudinal axis of the counterbalance arm. In the disclosed embodiment of a counterbalance arm, second parallel link is formed so that a pin extends through the longitudinal slot and a roller which engages an upper wall of the second parallel link to transfer this force to the second parallel link through the roller rather than through engagement of the pin with the upper wall of the slot. The component perpendicular to the longitudinal axis of the counterbalance arm creates a torque about the pivot axis at the proximal end of the counterbalance arm which is preferably substantially equal in magnitude to, but opposite in direction to, the torque induced by the weight.
Unfortunately, gas springs do not always obey Hooke's law precisely throughout the full stroke range, so it is difficult to design a counterbalance arm that cancels out the torque induced by the weight of the object suspended by the arm. Hooke's Law provides a linear equation relating the force exerted by an elastic medium to the displacement from equilibrium of the medium. In: gas springs the displacement can be represented by the stroke length of the piston. Hooke's Law states that force (F) of a spring is equal to the displacement of the spring from its equilibrium position (x) times a spring constant (k) or:F=kx.However, it has been found that gas springs are typically manufactured so that near the ends of their stroke lengths, the gas spring is no longer operating in the linear region of the displacement v. force curve.
In accordance with one embodiment of the disclosed surgical theater system, a counterbalance arm is provided which includes an adjustment pin set at an angle to the vertical for modifying the offset of the proximal end of a spring link from the proximal end of a first parallel link so that the proximal end of the spring link is not mounted at a fixed location relative to the pivot axis at the proximal end of the counterbalance arm. The angle that adjustment pin forms with the vertical introduces a mechanical disadvantage to the spring link/spring/slot combination to compensate for increased spring forces generated when the counterbalance arm is lowered from the raised to a lowered position.
In accordance with one embodiment, the counterbalance arm includes a first parallel link, a second parallel link, a spring link and a spring. The first parallel link is mounted at the proximal end to pivot about a proximal pivot axis and mounted at a distal end to pivot around a distal pivot axis. The second parallel link is mounted at its proximate end for pivotal movement about a second proximate pivot axis and at its distal end for pivotal movement about a second distal pivot axis. The displacement between the first and second proximal pivot axes and the displacement between the first and second distal pivot axes is substantially equal to maintain the parallel relationship between the first and second parallel links. The spring link is mounted at its proximal end to pivot about a third proximal axis and at its distal end to slide relative to the second parallel link. The third proximal axis extends through a pivot pin which is movable relative to the first proximal pivot axis to adjust the displacement between the third and first proximal pivot axes.
Additional features and advantages of the support arm for a surgical theater will become apparent to those skilled in the art upon consideration of the following detailed descriptions exemplifying the best mode of carrying out the apparatus as presently perceived.